From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking
Date: Thu, 08 Sep 2011 10:51:26 +0300
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To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Don Zickus <dzickus@redhat.com>, Peter Zijlstra <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>, Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Nick Piggin <npiggin@kernel.dk>, Marcelo Tosatti <mtosatti@redhat.com>, KVM <kvm@vger.kernel.org>, Andi Kleen <andi@firstfloor.org>, Xen Devel <xen-devel@lists.xensource.com>, Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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On 09/07/2011 10:09 PM, Jeremy Fitzhardinge wrote:
> On 09/07/2011 10:41 AM, Avi Kivity wrote:
> >>  Hm, I'm interested to know what you're thinking in more detail.  Can you
> >>  leave an NMI pending before you block in the same way you can with
> >>  "sti;halt" with normal interrupts?
> >
> >
> >  Nope.  But you can do
> >
> >     if (regs->rip in critical section)
> >             regs->rip = after_halt;
> >
> >  and effectively emulate it.  The critical section is something like
> >
> >      critical_section_start:
> >          if (woken_up)
> >              goto critical_section_end;
> >          hlt
> >      critical_section_end:
>
> Hm.  It's a pity you have to deliver an actual interrupt to implement
> the kick though.

I don't think it's that expensive, especially compared to the 
double-context-switch and vmexit of the spinner going to sleep.  On AMD 
we do have to take an extra vmexit (on IRET) though.

> >>
> >>  I was thinking you might want to do something with monitor/mwait to
> >>  implement the blocking/kick ops. (Handwave)
> >>
> >
> >  monitor/mwait are incredibly expensive to virtualize since they
> >  require write-protecting a page, IPIs flying everywhere and flushing
> >  tlbs, not to mention my lovely hugepages being broken up mercilessly.
>
> Or what about a futex-like hypercall?
>

Well we could have a specialized sleep/wakeup hypercall pair like Xen, 
but I'd like to avoid it if at all possible.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.